Emphasize target is on remote host. List supported file types.

From the documentation it is not immediately clear that the 'target'
option refers to a location on the remote host. This change emphasizes that.
In addition to .sql files, .bz2 and .gz files are supported for dumps and
restores. This is now documented.
Merge pull request #6183 from gkoller/devel

mysql_db: Emphasize target is on remote host. List supported file types.